The field of invention relates to lithography in general and, more specifically, to the manufacturing of phase shift masks used in semiconductor processing.
Phase shift masks have received increased attention over the years as semiconductor device size continues to shrink. Phase shift masks involve the technique of passing light through transparent mask regions where some transparent mask regions shift the phase of the light through the mask as compared to other transparent mask regions. Thus light that passes through a phase shifting mask contains regions of light that are out of phase with respect to one another.
Traditionally, phase shift masks pass 180xc2x0 shifted light and unshifted (i.e., 0xc2x0 phase shift) light. This produces regions of light that are out of phase by 180xc2x0 with respect to each other. Due to deconstructive interference principles, no light intensity exists where these regions of light overlap on a substrate surface. That is, on the substrate surface where light from the two regions overlap, the 180xc2x0 shifted light region cancels out the 0xc2x0 shifted region. The overlap occurs due to the natural spreading of light after it passes through a transparent region of the mask. The spreading may also be referred to as fringing.
By strategically patterning the transparent regions on the mask that corresponds to shifted and non-shifted light such that overlap occurs at specific locations on the substrate surface; the resulting deconstructive interference may be used to enhance optical contrast on the substrate surface. Enhanced optical contrast results in the ability to produce smaller features (such as gates, contacts and metal lines) on the substrate surface. Thus phase shift masks may be used to produce smaller features on the surface of a substrate.
A problem with phase shift masks concerns the result of manufacturing defects in the transparent regions associated with the mask. These manufacturing defects are usually caused by imperfect etching of the transparent regions. A transparent region is etched in order to determine its phase shift as compared to other non etched transparent regions. Typically, etched transparent regions have an 180xc2x0 phase shift while non etched regions have a 0xc2x0 phase shift. The depth of the etch determines the amount of phase shift that occurs.
FIGS. 1a and 1b show an example of the type of defects described above. If an unwanted portion 102 of resist layer 101 or opaque layer 111 or other material remains on the transparent layer 103 during the etching of the transparent layer 103 an imperfection 104 in the transparent region 105 is formed. Since the depth 106 of the transparent region 105 controls the phase shift through the transparent region 105, imperfection 104 results in an incorrect phase shift for light that passes through imperfection 104. The defect 102 and 104 can extend completely over the transparent region 105.
U.S. Pat. No. 5,308,722 entitled xe2x80x9cVoting Technique For The Manufacture of Defect-Free Printing Phase Shift Lithographyxe2x80x9d issued on May 3, 1994 describes a voting technique that addresses the defect problem. A voting technique involves multiple etch steps in order to reduce the effects of a defect mechanism. For example, as shown in FIGS. 2a-2e, the xe2x80x9cone stepxe2x80x9d etch of FIGS. 1a and 1b is expanded to a xe2x80x9ctwo-stepxe2x80x9d etch (also referred to as a xe2x80x9ctwice votedxe2x80x9d etch).
After a first resist layer 201 patterning (FIG. 2a), transparent layer 203 etch (FIG. 2b) and resist layer 201 removal (FIG. 2c), a second resist layer 207 is patterned (FIG. 2d) prior to a second transparent region 205 etch (FIG. 2e). The consequence of the xe2x80x9ctwo stepxe2x80x9d etch is seen in the height 210 of the imperfection 204.
Whereas the height 110 of the xe2x80x9cone stepxe2x80x9d etch imperfection 104 is approximately equal to the final etch depth 106 of the transparent region 105; the height 210 of the two step imperfection 204 is approximately half of this depth 206. Voting techniques therefore take advantage of the random location of imperfections per resist patterning, transparent region etch and resist removal sequence. The result is lower imperfection height 210.
Prior art voting techniques are traditionally limited to a final etch depth 106, 206 that corresponds to a 180xc2x0 phase shift. As described ahead, however, the overall effects of imperfections 104, 204 on the devices (such as semiconductor devices) manufactured with phase shift masks may be further reduced by other final etch depth approaches.
An apparatus comprising a phase shift mask having a transparent region. The transparent region comprises an etched region of a transparent layer. The etched region has a final etch depth that corresponds to a designed for phase shift that is greater than 180xc2x0.